Methods and apparatus for multi-carrier communication systems with automatic repeat request (arq)

ABSTRACT

Hybrid ARQ is employed in a multi-carrier communication system for retransmission of erroneous packets by taking advantage of time/frequency/space diversity and by combining ARQ functions at physical layer and MAC layers, making the multi-carrier system more robust in a high packet-error environment.

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a continuation of, and incorporates by reference inits entirety, U.S. patent application Ser. No. 10/583,239, filed Oct.16, 2008, entitled “METHODS AND APPARATUS FOR MULTI-CARRIERCOMMUNICATION SYSTEMS WITH AUTOMATIC REPEAT REQUEST (ARQ),” which is a371 of PCT Application No. PCT/US2005/003889, filed Feb. 7, 2005,entitled “METHODS AND APPARATUS FOR MULTI-CARRIER COMMUNICATION SYSTEMSWITH AUTOMATIC REPEAT REQUEST (ARQ),” which claims the benefit of U.S.Provisional Patent Application No. 60/542,317, filed Feb. 7, 2004,entitled “METHODS AND APPARATUS FOR MULTI-CARRIER COMMUNICATION SYSTEMSWITH AUTOMATIC REPEAT REQUEST (ARQ).” This application also relates toPCT Application No. PCT/US2005/003518 titled “METHODS AND APPARATUS FOROVERLAYING MULTI-CARRIER AND DIRECT SEQUENCE SPREAD SPECTRUM SIGNALS INA BROADBAND WIRELESS COMMUNICATION SYSTEM,” filed Jan. 27, 2005, whichclaims the benefit of U.S. Provisional Application No. 60/540,032 filedJan. 29, 2004 and U.S. Provisional Application No. 60/540,586 filed Jan.30, 2004.

BACKGROUND

Automatic Repeat Request (ARQ) schemes are often used in packetcommunication systems to improve transmission reliability. Hybrid ARQ isa method that combines both FEC (forward error correction) and ARQ wherepreviously unsuccessful transmissions are used in FEC decoding insteadof being discarded. Hybrid ARQ enhances the effectiveness of FECdecoding and allows FEC blocks to be sent at high error rate operatingpoints (S. B. Wicker, Error Control Systems for Digital Communicationand Storage, Prentice-Hall, Inc., 1995).

One form of hybrid ARQ is “Chase” combining where the transmitterretransmits the same coded data packet (D. Chase, “Code Combing: Amaximum-likelihood decoding approach for combining an arbitrary numberof noisy packets,” IEEE Trans. on Commun., Vol. 33, pp.593-607, May,1985). The decoder at the receiver combines multiple copies of thistransmitted packet in a certain manner. Another form is calledincremental redundancy, where instead of sending simple repeats of thecoded data packet, progressive parity packets are sent in eachsubsequent transmission of the packet. The decoder then combines packetswith incremental information in an appropriate fashion and thereforedecodes the packet at a lower code rate.

Hybrid ARQ normally involves the functionality at the physical layer andcontrols the FEC encoding and FEC decoding functions using an embeddedphysical layer fast feedback channel for control signaling. At times,the physical layer hybrid ARQ-FEC blocks may be retransmitted for themaximum number of times without success. Therefore, it alone cannotprovide error free data delivery but permits operation at a lowersignal-to-interference-plus-noise ratio (SINR).

Medium access control (MAC) ARQ is an error control feature whichretransmits erroneous MAC packet data units (PDUs) in a flexible fashionto achieve error free data delivery. MAC-ARQ retransmissions may occurlong after original transmission and the retransmission may be segmentedand piggy backed on other MAC PDUs using the granularity of the definedARQ block size.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a basic structure of a multi-carrier signal in thefrequency domain, made up of subcarriers.

FIG. 2 illustrates a radio resource divided into small units in both thefrequency and time domains: subchannels and time slots.

FIG. 3 illustrates a single ARQ process where a first transmission of apacket has failed with an NACK feedback, and a second transmission ofthe packet (may or may not be of the same size) has succeeded with anACK feedback.

FIG. 4 depicts a system reserving at least one subchannel forretransmission of packets.

FIG. 5 depicts a case in which Packet p and q from same subscriber aretransmitted in Frame k. Packet p fails and Packet q succeeds. Packet pis retransmitted on a subchannel that was originally scheduled forPacket q in Frame k+m.

FIG. 6 depicts a case in which Packet p and q from same subscriber aretransmitted in Frame k. Packet p fails and is retransmitted on samesubchannel in Frame k+m.

DETAILED DESCRIPTION

systems are described herein. In particular, methods and apparatus aredevised to carry out retransmission of erroneous packets by takingadvantage of time/frequency/space diversity. In addition, a hierarchicalARQ scheme is designed to combine ARQ functionality at physical layerand MAC layers, thereby making the multi-carrier system more robust in ahigh packet-error environment.

The multi-carrier system mentioned here can be of any format such asOFDM, or Multi-Carrier Code Division Multiple Access (MC-CDMA). Thepresented methods can also be applied to downlink, uplink, or both,where the duplexing technique is either Time Division Duplexing (TDD) orFrequency Division Duplexing (FDD).

The following description provides specific details for a thoroughunderstanding of the various embodiments and for the enablement of oneskilled in the art. However, one skilled in the art will understand thatthe invention may be practiced without such details. In some instances,well-known structures and functions have not been shown or described indetail to avoid unnecessarily obscuring the description of theembodiments.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words “comprise,” “comprising,” and thelike are to be construed in an inclusive sense as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to.” Words using the singular or pluralnumber in this Detailed Description section also include the plural orsingular number respectively. Additionally, the words “herein,” “above,”“below” and words of similar import, when used in this application,shall refer to this application as a whole and not to any particularportions of this application. When the claims use the word “or” inreference to a list of two or more items, that word covers all of thefollowing interpretations of the word: any of the items in the list, allof the items in the list and any combination of the items in the list.

Multi-Carrier Communication System

The physical media resource (e.g., radio or cable) in a multi-carriercommunication system can be divided in both the frequency and timedomains. This canonical division provides a high flexibility and finegranularity for resource sharing.

The basic structure of a multi-carrier signal in the frequency domain ismade up of subcarriers. Within a particular spectral band or channel,there are a fixed number of subcarriers, which are of three types:

1. Data subcarriers, which carry information data;

2. Pilot subcarriers, whose phases and amplitudes are predetermined andmade known to all receivers and which are used for assisting systemfunctions such as estimation of system parameters; and

3. Silent subcarriers, which have no energy and are used for guard bandsand DC carrier.

The data subcarriers can be arranged into groups called subchannels tosupport scalability and multiple-access. The carriers forming onesubchannel are not necessarily adjacent to each other. Each subscribermay use part or all of the subchannels. The concept is illustrated inFIG. 1, which illustrates a basic structure of a multi-carrier signal inthe frequency domain, made up of subcarriers. Data subcarriers can begrouped into subchannels in a particular manner. The pilot subcarriersare also distributed over the entire channel in a specific manner.

The basic structure of a multi-carrier signal in the time domain is madeup of time slots to support multiple-access. The resource division inboth the frequency and time domains is depicted in FIG. 2, whichillustrates a radio resource divided into small units in both thefrequency and time domains: subchannels and time slots.

Adaptive modulation and coding (AMC) adjusts the modulation and codingscheme in response to various channel conditions. It can be controlledfor one individual subchannel or a group of subchannels. Table 1provides an example of the coding and modulation schemes in AMC andcorresponding spectral efficiency in bits/s/Hz.

TABLE 1 Examples of coding and modulation schemes in adaptive modulationand coding control. Modulation Scheme Code Rate Bits/s/Hz QPSK ⅛ ¼ QPSK¼ ½ QPSK ½ 1 16QAM ½ 2 16QAM ¾ 3 64QAM ⅔ 4 64QAM ⅚ 5

FIG. 3 illustrates a single ARQ process where the first transmission ofpacket 301 has failed with the NACK feedback 302, and the secondtransmission of packet 303 (may or may not be of the same size) hassucceeded with the ACK feedback 304.

The ARQ Scheme

In a multi-carrier system, multiple subchannels can be used to transmitpackets. Here, the hybrid ARQ scheme is used for at least one of thesubchannels. Without loss of generality, one such subchannel is herebydesignated as SC_(i). For each of the packets transmitted over SC_(i),the receiver performs a receiving process, based on the receivedinformation, which corresponds to the transmission process. Subsequentlythe receiver performs error detection on the received packet, and basedon the detection result, sends an acknowledgement (ACK or NACK) signal,via a return channel, to inform the transmitter whether the reception ofthis particular packet was successful (ACK) or not (NACK).

In one embodiment, a channel quality indicator (CQI), indicating channelconditions, is transmitted along with the ACK/NACK signal to assist theselection of a subchannel to be used for the retransmission of thefailed packet or the transmission of the next packet. The CQI is afunction of, e.g., the signal-to-noise ratio (SNR),signal-to-interference-plus-noise ratio (SINR), bit error rate, symbolerror rate, packet error rate, frame error rate, pilot signal powerlevel, signal mean square error, or any combination thereof, which aremeasured based on the previous packet(s). In another embodiment thechannel quality information transmitted along with the ACK/NACK signalcomprises channel measurements.

After the transmitter receives a NACK signal, it selects a differentsubchannel, e.g. SC_(i), to retransmit the failed packet since SC_(i)may have a different channel response and a different interference levelthan SC_(i), thereby creating frequency and time diversity effects thatcan be taken advantage of at the receiver to improve the performance. Atthe receiver, for the demodulation and decoding of the packet, thepreviously received signals that have been stored at the physical layerand the newly received retransmission signals may be combined.

In one embodiment, Chase combining is used where the soft samples of thesame packet from previous transmission(s) and the current retransmissionare combined coherently to provide additional diversity gain. In anotherembodiment, incremental redundancy is used where progressive paritypackets are sent in each subsequent transmission of the packet. Theretransmission process and the receiving process can continue until thepacket is successfully received or a pre-specified number ofretransmissions is reached.

The transmitter can reconfigure a subchannel for retransmission. Thisreconfiguration can be carried out in any combination of time,frequency, space, signal power, modulation, coding, or other signaldomains. For example, in case of orthogonal frequency divisionmultiple-access (OFDMA) signals, the transmitter can change thesubcarrier composition of a subchannel. The newly composed subchannelmay contain different subcarriers, as well as different training pilots,in terms of number, location, or other attributes.

In one embodiment, the transmitter randomly selects SC_(i) from thesubchannels available to the transmitter for the retransmission. Inanother embodiment, the transmitter, based on the information conveyedby CQI of all or some of the subchannels, selects a subchannel forretransmission in such a way that the system efficiency is optimized.For example, the subchannel with the best quality is assigned forretransmission of the packet that has failed multiple times.

In yet another embodiment, the system reserves at least one subchannelfor the retransmission of the packets. This process is illustrated inFIG. 4. In FIG. 4A, packets p and q from the same subscriber aretransmitted, in Frame k, where packet p fails to be accurately receivedby the receiver and packet q succeeds. As depicted in FIG. 4B, packet pis retransmitted on a reserved channel 402 in Frame k+m. In FIG. 4 onlyone reserved subchannel is illustrated, while multiple reservedsubchannels may be allocated in different embodiments.

Different embodiments take different measures to improve the channelquality of the reserved subchannels 402. In particular, in a multi-cellenvironment, a higher frequency reuse factor is used for reservedsubchannels 402 to reduce the impact of the inter-cell interference. Forinstance, when regular subchannels have a reuse factor of 1, thereserved subchannels 402 may have a reuse factor of 3. The transmittermay select SC_(i) randomly from the reserved subchannels 402, or selectSC_(i) with sufficiently high quality if the transmitter knows from theCQI information a quality of all or some of the reserved subchannels402.

In one embodiment, the transmitter uses a modulation/coding/power schemethat matches the channel quality of that subchannel(s), in which casethe retransmitted packet is fitted into the subchannel(s) by ratematching such as by repetition or puncturing.

In one embodiment, at least two subchannels are allocated for thetransmitter by the system. Upon receiving a NACK signal indicating theneed for retransmitting a packet, the transmitter swaps the transmissionof the two subchannels SC_(i) and SC_(i) and sends the retransmissionover SC_(i) and sends the packet originally scheduled for SC_(i) overSC_(i). This process is illustrated in FIG. 5. In FIG. 5A, packets p andq from the same subscriber are transmitted, in Frame k, wherein packet pfails and packet q succeeds. In this situation, as depicted in FIG. 5B,packet p is retransmitted in Frame k+m via the subchannel over whichpacket q was sent. No reserved channel is provided in this embodiment.

In one embodiment, the retransmission over SC, uses the same settings,such as modulation, coding, and power, as the previous transmission overSC_(i). When the packet size is different between the currenttransmission and previous transmission on SC_(i), rate matching is usedto fit the current retransmitted packet onto SC_(i).

In some cases it may be desirable for the transmitter to stay on theoriginal subchannel for the retransmission. This process is illustratedin FIG. 6. In FIG. 6A packet p and q from the same subscriber aretransmitted in Frame k, wherein packet p fails. As depicted in FIG. 6Bpacket p is retransmitted on the same subchannel in Frame k+m. If thereare no other subchannels available to the transmitter at the time ofretransmission, the transmitter selects SC_(i)=SC_(i). If thetransmitter has the knowledge about the quality of all or some of thesubchannels and finds that the quality of SC_(i) is good or better thanthe rest of the available subchannels, it again selects SC_(i)=SC_(i).

In yet another embodiment, the channel quality of SC_(i) is good and themodulation/coding index is high (16QAM or 64QAM), so the transmitterselects SC_(i)=SC_(i). It should be noted, however, that the transmittermay lower down the modulation/coding scheme in the case ofretransmission based on the channel quality report about the previouslytransmitted packets on the same subchannel.

In one embodiment, multiple subscribers may share one subchannel, forexample, through time division multiplexing. Then multiple ARQprocesses, each corresponding to a subscriber, can be carried out inparallel. The above described methods of retransmission are applicableto this embodiment.

In some embodiments, higher layer messaging dictates whichretransmission process to be used. In other embodiments, informationabout a retransmission process is embedded in headers of retransmittedpackets.

In one embodiment, a hierarchical ARQ process is implemented for apacket stream. The process includes an outer loop and at least one innerloop. The outer loop operates at a higher layer, for example, at theradio link protocol (RLP) layer, with a traditional ARQ approach such assliding window selective-retransmission ARQ. The inner loops operate atlower layers, for example, the physical layer, with one of the hybridARQ methods described in the above embodiments.

The parameters for both the outer and the inner loop can be changeddepending on the application or the unit processing capabilities. Forexample, the number of retransmissions within the inner loops is setsmaller for delay-sensitive applications than for otherdelay-insensitive applications using TCP (Transmission ControlProtocol). In one embodiment, the outer loop is removed for a UDP (UserDatagram Protocol) packet stream such as VoIP (Voice over InternetProtocol) packets.

In one embodiment, the receiver combines the originally transmittedsignal and the retransmitted signal, which are transmitted over the sameor different subchannel, to detect the data packet.

The above detailed description of the embodiments of the invention isnot intended to be exhaustive or to limit the invention to the preciseform disclosed above or to the particular field of usage mentioned inthis disclosure. While specific embodiments of, and examples for, theinvention are described above for illustrative purposes, variousequivalent modifications are possible within the scope of the invention,as those skilled in the relevant art will recognize. Also, the teachingsof the invention provided herein can be applied to other systems, notnecessarily the system described above. The elements and acts of thevarious embodiments described above can be combined to provide furtherembodiments.

All of the above patents and applications and other references,including any that may be listed in accompanying filing papers, areincorporated herein by reference. Aspects of the invention can bemodified, if necessary, to employ the systems, functions, and conceptsof the various references described above to provide yet furtherembodiments of the invention.

Changes can be made to the invention in light of the above “DetailedDescription.” While the above description details certain embodiments ofthe invention and describes the best mode contemplated, no matter howdetailed the above appears in text, the invention can be practiced inmany ways. Therefore, implementation details may vary considerably whilestill being encompassed by the invention disclosed herein. As notedabove, particular terminology used when describing certain features oraspects of the invention should not be taken to imply that theterminology is being redefined herein to be restricted to any specificcharacteristics, features, or aspects of the invention with which thatterminology is associated.

In general, the terms used in the following claims should not beconstrued to limit the invention to the specific embodiments disclosedin the specification, unless the above Detailed Description sectionexplicitly defines such terms. Accordingly, the actual scope of theinvention encompasses not only the disclosed embodiments, but also allequivalent ways of practicing or implementing the invention under theclaims.

While certain aspects of the invention are presented below in certainclaim forms, the inventors contemplate the various aspects of theinvention in any number of claim forms. Accordingly, the inventorsreserve the right to add additional claims after filing the applicationto pursue such additional claim forms for other aspects of theinvention.

1.-20. (canceled)
 21. A method for a transmitter in a communicationsystem, the method comprising: allocating first and second subchannelsfor signal transmissions to a receiver in the communication system, thesubchannels containing Orthogonal Frequency Division Multiplexing (OFDM)subcarriers; transmitting, at a first transmission time, a first datapacket over the first subchannel and a second data packet over thesecond subchannel; receiving a signal from the receiver indicating anerror in the first data packet received by the receiver; andtransmitting, at a second transmission time, a redundancy version of thefirst data packet over the second subchannel and a third data packetoriginally scheduled for the second subchannel over the firstsubchannel, wherein the first subchannel and second subchannels havedifferent spatial configurations.
 22. The method of claim 21, whereinthe transmission over the first or second subchannel uses an adaptivemodulation and coding scheme.
 23. The method of claim 21, wherein thetransmission over the first or second subchannel uses a rate-matchingscheme.
 24. The method of claim 21, wherein the redundancy version ofthe first data packet is a progressive parity packet of the first datapacket.
 25. The method of claim 21, wherein the signal from the receiverindicating an error in the first data packet is a negativeacknowledgement (NACK) from the receiver.
 26. The method of claim 21,further comprising receiving a channel quality indicator (CQI), channelmeasurements, or both from the receiver.
 27. A method for a receiver ina communication system, the method comprising: receiving resourceallocation information of first and second subchannels from atransmitter in the communication system, the subchannels containingOrthogonal Frequency Division Multiplexing (OFDM) subcarriers; receivingfrom the transmitter, at a first reception time, a first data packetover the first subchannel and a second data packet over the secondsubchannel; transmitting a signal to the transmitter indicating an errorin the received first data packet; receiving from the transmitter, at asecond reception time, a redundancy version of the first data packetover the second subchannel and a third data packet originally scheduledfor the second subchannel over the first subchannel, wherein the firstsubchannel and second subchannels have different spatial configurations;and combining the data received over the first subchannel at the firstreception time and the data received over the second subchannel at thesecond reception time to recover the first data packet.
 28. The methodof claim 27, wherein the reception of the data packets over the first orsecond subchannel are performed in accordance with an adaptivemodulation and coding scheme.
 29. The method of claim 27, wherein thereception of the data packets over the first or second subchannel areperformed in accordance with a rate-matching scheme.
 30. The method ofclaim 27, wherein the redundancy version of the first data packet is aprogressive parity packet of the first data packet.
 31. The method ofclaim 27, wherein the signal to the transmitter indicating an error inthe received first data packet is a negative acknowledgement (NACK). 32.The method of claim 27, further comprising transmitting a channelquality indicator (CQI), channel measurements, or both to thetransmitter.
 33. A transmitter for a communication system, thetransmitter comprising: an apparatus configured to allocate first andsecond subchannels for signal transmissions, the subchannels containingOrthogonal Frequency Division Multiplexing (OFDM) subcarriers; anapparatus configured to transmit, at a first transmission time, a firstdata packet over the first subchannel and a second data packet over thesecond subchannel; an apparatus configured to receive a signalindicating an error in the first data packet received by a receiver; andan apparatus configured to transmit, at a second transmission time, aredundancy version of the first data packet over the second subchanneland a third data packet originally scheduled for the second subchannelover the first subchannel, wherein the first subchannel and secondsubchannels have different spatial configurations.
 34. A receiver for acommunication system, the receiver comprising: an apparatus configuredto receive resource allocation information of first and secondsubchannels from a transmitter in the communication system, thesubchannels containing Orthogonal Frequency Division Multiplexing (OFDM)subcarriers; an apparatus configured to receive from the transmitter, ata first reception time, a first data packet over the first subchanneland a second data packet over the second subchannel; an apparatusconfigured to transmit a signal to the transmitter indicating an errorin the received first data packet; an apparatus configured to receive,at a second reception time, a redundancy version of the first datapacket over the second subchannel and a third data packet originallyscheduled for the second subchannel over the first subchannel, whereinthe first subchannel and second subchannels have different spatialconfigurations; and an apparatus configured to combine the data receivedover the first subchannel at the first reception time and the datareceived over the second subchannel at the second reception time torecover the first data packet.